Protection for electron tubes



P 1946. R. F. HOLTZ 2,407,8 PROTECTION FOR ELECTRON TUBES Filed Oct 31,1944 22 A; I I Z Z p iECWf/ER m M04765 JWPPZ) o I N V EN TOR.

Patented Sept. 17, 1946 UNETED 3'5 TES T s? race PROTECTION FOR ELECTRONTUBES Delaware Application Gctober 31, 1944, Serial No. 561,280

This invention relates to the protection of electron tubes, and moreparticularly to the protection of such tubes from damage which may becaused by abnormal current flow in two of its x na circu t Whileover-current and under-current relays for this purpose are well-hn-own,it is often necessary to afford protection at the sometime against thepossibility of abnormal current flow in two associated circuits of atube; and it is sometimes undesirable, on grounds oi cost, and moreparticularly of space, to use two relays. Thus, in the generation ofhigh frequency heating cur-- rents, anode current rises steadily duringthe heating cycle; towards the end of the cycle, and

sometimes in the middle of it, a substantial increase of anode currentmay occur which, if continned, will cause damage to the tube, and may atleast cause it to lose emission. There is also the possibility ofabnormal current flow in the grid circuit, and this may be either inexcess of or below the normal operating grid current. An abnormal flowof grid current will not always be reflected in a change of anodecurrent, particularly when the electrical characteristics of the loadarechanging; and it may be overlooked by an unskilled operator.

It is, therefore, an object of the invention to provide an improvedmethod of and means for protectingan electron tube from damage which maybe caused by abnormal current flow.

Another object is to provide an improved, compact and unitary relay forthe prevention of abnormal current flow two associated circuits of anelectron tube.

A further object of the invention is to provide improved means forprotecting automatically an electron tube from damage due to abnormalcurrent flow whic may not be evident to the oporator.

These objects are achieved in one embodiment of the invention byinserting a resistor in each of two associated circuits of a tube, insuch a mannor as to procure equal and opposing voltages in each of theresistors. These voltages depend on the normal flow of current in thecircuits in which they appear. A relay adapted to sic-energize the tubeor otherwise control its operation is connected with its actuatingwinding in shunt with the resistors. Any change, therefore, in the ratioof current in the two circuits will result in a voltage being impressedon the winding of the relay, and actuation of the relay.

Figure l of the accompanying drawing is a schematic representation of atriode with its as- 4 Claims. (Cl. 315-1l9) 2 s-ociated circuits andembodying the rela of the present invention, and

Figure 2 is a schematic drawing similar to Figure 1, but showing theapplication of the invention to a tube with more than three electrodes.

An electron tube ii) i represented as having a cathode l2, a grid Itandan anode l6. The anode circuit iii of the tube includes a source ofpower, indicated generally at 29, for the anode, and the total impedanceof this circuit (including impedance of the circuit within the tubeitself) is represented by the block marked Zp. Similarly, the block Zgrepresents the total impedance of the grid circuit 22 of the tube.

A resistor AC is connected partly in the anode circuit and partly in thegrid circuit; the resistor is also connected at an intermediate tappingpoint B to the cathode l2 by a conductor 2,4. Point B is so chosen on ACthat the voltage drop from A to B is equal to that from B to C. Anexample will illustrate what value toselectfor the resistor AC and howto determine the point B.

An oscillator in commercial use in high frequency industrial heatingconsists of two RCA Type 892R tubes connected in parallel. These tubesare very expensive, and it'is highly desirable that they should not beoperated at other than their proper ratings, particularly for any lengthof time. In normal operation, D. Generic current should be of the orderof 3.5 amps, and grid current approximately 35,0 m. a. If :a loss ofvolts can be tolerated in the anode circuit due to the relay, then BCmay have a resistance of approximately 30 ohms. Since the voltage dropacross AB must equal that across BC, the resistance of AB will beapproximately 3.00 ohms, and the total resistance of AC about 330 ohms.

By-pass condensers 32 and 34 are connected in shunt with the resistor-AB and BC respectively, so that the voltage appearing across these tworesistors shall not be afiected by current of radio frequency flowing ineither of the circuits 53 or 22.

An actuating winding 25 or .a relay 2% is connected in parallel with theresistor AC, current in the secondary circuit to of the relay beingemployed to cut off the supply of power from the source 26. In practice,the secondary relay circuit 3! i connected with the various auxiliarycontrols or interlocked safety devices which automatically cut off thesupply of power to the anode i 5 when, for example, any of the doorsgiving access to the apparatus are opened.

If current in the anode circuit l3 rises to 4 amps, the voltage dropacross BC rises to volts. If, therefore, it is desired that anodecurrent shall not exceed 4 amps, the relay 28 is so designed as to beactuated when a voltage of 20 volts appears across the Winding 26. Inthat event, and if anode current remained unchanged, the relay wouldalso be actuated if grid current exceeded 400 m. or fell below 260 m.Thus, operation of the relay depends on a variation in the ratio ofcurrent normally flowing in the two associated circuits of the tube,this in turn causing a variation in the ratio of the voltage dropsacross AB and BC. The circuit illustrated constitutes abalanced bridgecircuit with the resistors AB and BC forming two adjacent arms of thebridge, the impedances'Z and Zg constituting the other two adjacent armsand the winding 26 constituting a conjugate arm of the bridge.

Many variations of the embodiment described are within the scope of theinvention. It is not limited to the use of an electron tube as an 05-cillator, but may be employed in an amplifier, or wherever the ratio ofcurrent normally flowing in two circuits of a tube is subject tovariation. The invention is equally applicable to tubes which have morethan three electrodes, in which event theresistors AB and BC may becontained in the two circuits of the tube whose current is most likelyto vary. Fig. 2 illustrates the application of the invention to atetrode, parts which are common to Figs. 1 and 2 being designated-by thesame reference numerals. In Fig. 2 the tube IB- is shown as having ascreen grid electrode 36 in addition to the three electrodes mentionedin connection with Fig. 1. A circuit 38 connects this electrode to thecathode l2 and includes a source of power 48 for the screen grid. If thecircuits 22 and 38 are to be protected from an abnormal flow of current,the resistor ABC is connected in 7 these two circuits in the mannerindicated above.

Other departures from the embodiments illustrated in Figs. 1 and 2 mayinclude the use of a gas-filled tube or other form of relay in place ofthe mechanical relay 28, and the utilization of current in the secondaryrelay circuit 30 to give an alarm or perform some other protective orcontrol operation in place of, or in addition to de-energization of thetube In.

It should be noted that'the relay will not be actuated if both gridcurrent and anode increase or decrease simultaneously by such amountsthat .the balanceof potential across AB and BC is not sufii'cientlydisturbed to impress the required actuating potential on the winding 26.On the other hand, the arrangement here described is more effective thana prior art relay if one of the two currents increases while the otherdecreases.

There has thus been described a simple method of and means forprotecting an electron tube from damage due to an abnormal flow ofcurrent in its associated circuits. Two voltages representative ofcurrent in two of the tube circuits are balanced, and any unbalance ofthese voltages is 4 used to create a third voltage which is employed, inturn, to de-energize the tube.

I claim as my invention:

1. A system for protecting an electron tube, which has external grid andanode circuits, from damage due to an abnormal flow of current in eitherof said circuits, said system comprising a resistor connected in each ofsaid circuits to form a balanced bridge circuit on normal flow ofcurrent in said circuits, said resistors forming two of the arms of saidbridge, and the remaining total impedances of each of said circuitsforming the other two arms of said bridge, means for adjusting therelative values of said resistors, and a relay for de-energizing saidtube having its actuating winding connected across opposite points ofsaid bridge, whereby on abnormal flow of current in either of saidexternal circuits the balance of said bridge will be disturbed and saidrelay actuated to de-energize said tube.

2. Apparatus for protecting an electron tube from damage due to anabnormal flow of current in external grid and anode circuits, whichcomprise means for deriving a potential proportional to current flowingin one of said circuits, means for deriving a second potential equal inamount and opposite in direction to said first potential andproportional to current flowing in another of said circuits, means foradjusting the balance point of said potentials, and means responsive toa variation in the ratio of 'said potentials for deenergizing said tube.

3. An electron tube having at least a cathode, an anode and a grid, anexternal circuit connecting said cathode to said anode, 'a resistor insaid circuit of such value as to establish thereacross, by reason ofnormal current flow in said circuit, a voltage drop of predeterminedamount, a second external circuit connecting said grid to said cathode,a resistor in said second circuit of such value as to establishthereacross, by reason of norma1 current flow in said circuit, a voltagedrop equal to that of said first-mentioned resistor, means for adjustingthe relative values of said i resistors, a capacitor connected inparallel with each of said resistors, and means responsive to aninequality of said voltage drops for de-energizing said tube.

4. A unitary device for protecting a generator of high frequency heatingcurrent, which has associated grid and anode circuits, from damage dueto abnormal flow of current in either of said circuits, said devicecomprising a resistor, a part of which is so connected in one of saidcircuits and the remainder of which is so connected in the other of saidcircuits that on normal flow of current in each of said circuits thevoltage drops across each of said parts of the resistor is substantiallyequal, means for adjusting the relative m values of said parts of saidresistors, and a relay for dc-energizing said generator and having itsactuating winding connected in parallel with said resistor.

ROBERT F. HOLTZ.

